It is important in the practice of functional magnetic resonance imaging (fMRI), as well as many other visual experiments, to provide visual stimulus having precisely controlled and well defined start times and presentation durations. In some instances, the diagnostic procedure performed with the MRI is used to evaluate a patient's response to specific visual stimuli. The operator sends a series of images to a screen which is seen by the patient during the MRI procedure and the patient's responses are included in the MRI report.
Three important parameters must be controlled for predictable visual stimulus: color balance, brightness, and stimulus duration. Both the cathode ray tube (CRT) and the liquid crystal display (LCD) in conjunction with a projection lamp are used as image generation projectors.
The timing of a CRT is synchronized by the frame rate (also called the vertical deflection frequency). The frame rate is the time required for the raster to completely scan the screen. Typical frame rates are in the 60-150 Hz range. To write a complete frame, only discrete multiples of the time required for the raster to complete a frame are acceptable. Further, a new image can only begin at the beginning of a new frame. Even when these restrictions are observed and carefully controlled, additional time errors will be present. Generally, the raster is written from the top of the screen to the bottom. Therefore a small image object near the top of the screen will be seen sooner than an image at the bottom of the screen.
For the conventional LCD-lamp projector the limitations imposed by the frame rate described for the CRT are still present. Durations which are multiples of the frame rate are achievable. But the LCD-lamp projector has additional limitations. Because the conventional LCD has a relatively slow response time, frame rates are usually limited to 60 Hz. It may take several frames for an image to fully appear.
The LCD lamp projector also can have other time errors. The VGA signal supplied by a computer is usually an analog signal. But the LCD is driven with digital electronics. As a result, the signal undergoes an analog to digital conversion process inside the projector. This conversion process decouples the timing synchronization pulses inside the incoming VGA signal from the timing of when an image is displayed. An incoming signal could just miss being sampled by the analog to digital converter and have to wait until the next frame before being displayed. Terminating the display of an image contains the same indeterminacy. Delays from 7-40 ms have been measured for both turn on and turn off of an image.
Color balance can be achieved to some extent with the CRT. Many devices allow adjustment of the maximum intensity of the electron beams directed at the red, green and blue phosphor dots. The relative intensity of these dots (called sub-pixels) affects the perceived pixel color. Each pixel (usually a group of one each of a red, green, and blue sub-pixel) is simultaneously stimulated sequentially in the moving dot raster process.
The difficulty with color control in a CRT is two-fold: 1) the relationship of color intensity to electron beam intensity changes as the phosphors age; and 2) the electron beam intensity to brightness transfer function is non-linear. Color calibration with a CRT usually consists of applying a video signal set to supply a maximum emission of the red, green and blue electron beams. The color perceived when these maximum conditions are present is called the “white point.” This ratio of red, green and blue brightness is only valid at the test electron emission intensities. As stated above, each phosphor has a unique non-linear electron beam intensity to brightness transfer function. In an attempt to correct for this non-linearity, a number of intensity points are measured for each color and a set of γ-correction curves are generated. The γ-correction curves produce an approximate correction factor for each color. The process of white point and γ-correction adjustment must be repeated regularly, not only because of phosphor aging, but also because of environmental factors such as ambient lighting conditions and stray magnetic and electromagnetic fields.
The LCD lamp projector has a more limited ability to control color balance than even the CRT. Illumination is usually provided by an incandescent light source with a color temperature in the range of 3000-3800 K. This color temperature changes with supply voltage and lamp aging. An LCD lamp projector in its generic form consists of an array of red, green and blue filters positioned over liquid crystal variable shutter sub-pixels. The white point is determined by the lamp color temperature, the red, green, and blue filter characteristics, the LCD transmission characteristics, as well as other optical factors. None of these factors can be adjusted during use to produce a different white point.
A pseudo change in white point can be accomplished by decreasing the maximum allowable transmission (minimum transparency) for one or more of the sub-pixel color arrays. But by decreasing the maximum allowable transmission for a color, the contrast ratio range for the adjusted sub-pixel array is also reduced. Color calibration must also be performed after the lamp is “broken in” and after the map has warmed up to stabilize.
Brightness can be varied in a CRT but not in a linearly predictable fashion. As such, changes in brightness require calibration. Brightness adjustment in a LCD projector is limited if it exists at all. Lamp power dissipation can sometimes be varied a small amount. Lamp power changes will alter the color temperature as well as affect the lamp life.
A problem with introducing conventional audio or video signals into an MRI apparatus is that the device is based upon the use of radio frequency which will disrupt signal modulation. Further, the inner area of the bore produces a magnetic field which will draw metal items when magnetized. For this reason, the audio or video signal must be in a form that is not affected by the radio frequency and transmission by a mechanism that is not easily magnetized. This problem has been eliminated in the prior art by providing a fiber optic connection between the shielded MRI room and a remote location housing the elements of the system. An example of such a device is seen in U.S. Pat. No. 5,414,459, the disclosure of which is herein incorporated by reference.
Thus, what is lacking in the art is an LCD projection system adapted for use in an MRI environment which allows for precise control of the display parameters in order to provide a precise visual stimulus via a fiber optic connection.